A power supply circuit is used to supply power to a device. The power supply circuit may be modulated by a floating voltage source to drive an output voltage. When a high voltage output is required, the maximum modulation range is limited by the breakdown voltages of transistors used in the power circuit. To increase the modulation range, high voltage transistors can be used. However, high voltage transistors will increase the process costs. In addition, when the high voltage transistors are used in the power circuit, the high voltage transistors frequently fail to respond to a low input voltage.